FIG. 1 is a block diagram of a signal-drive current feedback circuit for a lamp 130 (or plural lamps) of a conventional backlight module 100. The backlight module 100 outputs a voltage (labeled AC in FIG. 1) from a control integrated circuit (IC) 110. The voltage AC is converted by a transformer 120 to a driving voltage AC′ for driving the lamp 130. The lamp 130 is a single-drive lamp (which is driven on one side of the lamp, with the other side of the lamp grounded as depicted in FIG. 1. The backlight module 100 has a feedback circuit 140 that detects lamp current Id flowing to the lamp 130. The feedback circuit 140 is coupled to one end of a secondary coil 122 of the transformer 120, and provides a feedback voltage Vfb to the control IC 110 accordingly. The control IC 110 changes the voltage AC according to comparison of the feedback voltage Vfb and a reference voltage Vref, and thus adjusts the driving voltage AC′ so that the luminance of the lamp 130 can be maintained at a target value.
A high-voltage end HE of the single-drive lamp 130 is coupled to the secondary coil 122 of the transformer 120, while a low-voltage end LE of the lamp 130 is coupled to the ground voltage. Furthermore, the feedback circuit 140 has one end coupled to the secondary coil 122 and the other end coupled to the grounded low-voltage end LE of the lamp 130. To allow a more stable lamp current Id to be detected by the feedback circuit 140, the high-voltage end HE of the lamp 130 is coupled to an additional capacitor C. With such a feedback circuit connection, since a part (Ic) of the lamp current Id flows through the capacitor C, the current actually driving the lamp 130 is a remaining portion Id′ of the current Id (in other words, Id′=Id−Ic). Therefore, the current Id detected by the feedback circuit 140 is not the actual current Id′ for driving the lamp 130, which reduces the detection accuracy of the feedback circuit 140.
Additionally, resistor devices (not shown) of the conventional feedback circuit 140 are unable to withstand the high voltages (thousands of volts) that are typically associated with the high-voltage end HE of the lamp 130. As a result, feedback circuit 140 is usually connected to the low-voltage end LE of the lamp 130 (through the ground connection depicted in FIG. 1). Such a feedback circuit 140 cannot usually be used with floating lamps (or dual-drive lamps) used in some conventional backlight modules.